In a two-stroke engine, the reciprocating movement of the piston opens and closes the exhaust and transfer ports. After combustion has occurred, the piston moves downwardly, uncovering the exhaust port, and allowing exhaust gases to exit the cylinder. When this happens a pressure wave, commonly called a blowdown pulse, is created on the exhaust side of the cylinder. This pulse as it travels down an exhaust pipe with expanding section creates reflections having a negative magnitude back towards the cylinder. This creates a pressure wave which helps to suck the exhaust gases out of the combustion chamber and a fresh charge of air into the combustion chamber or, as is the case in carburated engines, a fresh mixture of air and fuel. Once all of the exhaust gases have been sucked out of the combustion chamber, some of the fresh charge may get sucked out as well. This is known as the suction pulse.
It was discovered that by attaching a pipe to the exhaust port, the pressure wave would bounce from the end of the pipe and return to the exhaust port. The returning pressure wave pushes the fresh charge back into the combustion chamber before the exhaust port closes, filling it to greater pressures than could normally be achieved. This is known as the plugging pulse.
However, since the pressure waves are generated at the same frequency as the engine is turning, a pipe of a given length will only work over a narrow engine speed range. At engine speeds below that range, the pressure wave returns too soon and bounces back out of the exhaust port. At engine speeds above that range, the pressure wave returns too late because the exhaust port is already closed.
As a general rule, shorter pipes are effective at higher engine speeds, and longer pipes are effective at lower engine speeds.
It was later discovered that by adding a diverging section at the beginning of the pipe and a converging section at the end of the pipe, that the return pulse, although not as strong, is longer, and is therefore more likely to return while the exhaust port is opened. Such pipes are known as tuned pipes and are effective over a broader speed range.
The shape and length of the tuned pipe is based on various factors including the engine type, exhaust temperature, and desired engine operating range. The tuned pipe is “tuned” to be most efficient during that desired engine speed operating range as it cannot be efficient in all ranges.
In multi-cylinder engines having multiple tuned pipes, conduits are sometimes provided to communicate the tuned pipes together. By doing this, the blowdown pulse of one pipe can be used to “plug” the exhaust port associated with another cylinder. By overlapping the blowdown pulses this way, the engine speed range over which the tuned pipes are effective is broadened. This is known as intra-cylinder plugging.
However, intra-cylinder plugging becomes less effective as the number of cylinder is reduced, as there is less of an overlap between the opening of the exhaust ports.
Thus, while current exhaust systems having tuned pipes are effective over a certain engine speed range, there exists a need to provide an engine exhaust system which is effective over a broader range of engine speeds.